A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to project the pattern of the patterning device on the appropriate position on the substrate, accurate knowledge of the instantaneous position of the patterning device and the substrate is desired. In order to obtain this information, the lithographic apparatus is provided with one or more position measurement systems. The most commonly used position measurement systems include interferometer based measurement systems and encoder based measurement systems. The latter generally includes a sensor arranged to co-operate with a grating or pattern enabling the relative position of the sensor and the grating to be determined. In general, such an encoder based measurement system may be used for determining the position of a stage apparatus in a lithographic apparatus. In principle, such an encoder based measurement system provides information regarding a relative displacement of the sensor relative to the pattern when the sensor is moved from one position to an other. As an example, the pattern may be a periodic pattern of lines arranged adjacent to each other along the displacement direction, during the displacement, the sensor counts the number of lines that have passed (note that the individual lines of the pattern, in general, will extend in a direction substantially perpendicular to the displacement direction). Such encoder based measurement systems have been expanded to provide additional information regarding the position of the sensor relative to the grating. Examples are the use of a two-dimensional grating (e.g. including a checkerboard pattern) to provide positional information in two dimensions. Other configurations of the basic encoder measurement system as described above may include a second pattern in order to enable an absolute position measurement. As an example, the pattern may include a reference mark at some position, detection of this mark by the sensor enables a reference position to be set by the sensor.
In case two positional quantities are measured, the grating design tends to become complex, e.g. a checkerboard pattern rather than a pattern of lines, or the inclusion of a reference mark at a specific position within a pattern of lines. In the latter case, the accuracy of the linear grating may be affected by the introduction of e.g. a reference mark. As such, the manufacturing of the grating or pattern may be difficult.